The present invention relates to a new structure for high-power, low-loss microwave filters and a synthesis technique to implement same in a waveguide realization.
When considering a band-pass filter specification, the first steps that must be taken involve the generation of a filtering function. This is a purely mathematical process, the inputs to which include such parameters as out-of-band rejection corner points, inband group delay flatness etc which are normally supplied with the specification. The outcome is usually a ratio of two finite-degree polynomials, the evaluation of which yield rejection/return loss/group delay vs frequency curves which fit the specification.
The next step is to convert the purely mathematical filtering function into a low-pass prototype network of electrical elements such as capacitors and transmission lines, the electrical analysis of which will show characteristics equal to those resultant from the mathematical evaluation of the filtering function. The configuration of the network is arranged in a manner so that each element of the network corresponds with the equivalent element of the structure that will eventually be constructed to realize the filtering function. For microwave band-pass filters the configuration and topology of this low-pass network are important because mechanical constraints severely limit the variety of networks that can be realized with the cavities and irises of a microwave filter. FIG. 1 shows an 8th degree example of the most commonly used type of prototype network known as the cross-coupled double array and FIG. 2 shows an equivalent rectangular waveguide structure known as the folded configuration as it would be realized in accordance with the prior art.
The waveguide structure of FIG. 2 consists of two identical conventional direct-coupled 4-cavity filters with shunt reactive irises K between adjacent cavities, cross-coupled by small apertures K' in the common narrow wall. The shunt capacitors C.sub.1, C.sub.2 . . . C.sub.8 in FIG. 1 are realized in the electrical lengths of the cavities. If these waveguide elements are correctly dimensioned according to the values of their corresponding parameters in the low-pass network of FIG. 1, the waveguide structure will yield an electrical performance very similar to that embodied within the original filtering function.
With the increase in capacity and complexity of satellite telecommunication and broadcast repeaters over the past decade, the degree of linearity and selectivity specified for the filters within the repeaters has tightened considerably. In addition, as the RF power amplifiers become more powerful (particularly in TV broadcast missions), the insertion loss of the output multiplexer filters has to be minimized. In order to meet these requirements of linearity and selectivity specialized filtering functions have been developed, most commonly pseudo- and canonic elliptic functions. Each of these function types offers considerable advantages in terms of reductions in signal distortion, adjacent channel interference and insertion loss. Referring particularly to the networks of FIGS. 1 and 2, these specialized functions are realized with the cross-couplings K' which may be positive, zero or negative depending on the type of filtering function employed. In general the forward couplings K are positive and never zero. For purely linear-phase filters, the cross-couplings K' are either positive or zero, but for elliptic or combined linear-phase/elliptic types the cross-couplings K' become mixed-sign. The realization of the mixed-sign cross-coupling in waveguide is not a problem if the TE.sub.10 mode resonance in rectangular or square waveguide, or the TE.sub.11 mode in cylindrical guide are employed. With judicious placement of the cross-coupling irises in the rectangular or square waveguide structure, or using dual-moding techniques within the TE.sub.11 mode cylindrical cavity, all the specialized filtering functions mentioned above may be realized. However it was also mentioned above that with high power channels it is essential to minimize the loss of output multiplexer filters since failure to do so would result in cooling problems and have consequences in weight, reliability and signal distortion. For input filters, particularly those that combine channels before amplification, low loss is essential to minimize system noise figure.
With these points in mind it becomes advantageous to use the high Q (that is low-loss) TE.sub.01 mode cylindrical cavity resonance. Implicit in the high Q property is a lage cavity volume which renders construction easier at millimeter wave frequencies and a lower sensivity to manufacturing tolerances. A filter constructed using the TE.sub.01 cavity suffers from two major problems:
(a) negative couplings are only achievable with a complicated three-dimensional configuration; and
(b) the TE.sub.01 resonance is a higher order resonance and degenerate with it is the unwanted TM.sub.11 resonance.
The first attempt to construct an elliptic function filter was by Atia and Williams of Comsat Laboratories in 1976. They achieved the mixed-sign couplings with a complicated two-layer mechanical arrangement, and then "suppressed" the degenerate mode with dielectric cubes inside the cavity (actually these cubes shifted the frequency of the degenerate mode out of the filter's passband). The introduction of such dielectrics inevitably increases the loss of the cavity thereby eliminating the chief reason for using the solution. This is the only case known to the applicants of an attempt made to realize a filtering function with TE.sub.01 cavities.
The problems discussed above have been solved by the present invention which provides a new approach.